The present invention pertains generally to a distance measuring device for a camera, and more particularly to an active-type distance measuring device which projects a beam of light onto an object to be photographed and receives light reflected by the object to determine the distance to the object.
There are various active-type distance measuring devices know in the art. Typically, when an object to be photographed is not directly in the center of the photographic field of a camera, the object reflects only a portion of the light beam projected by a near infrared light emitting element (IRED), resulting in beam loss. This beam loss results in an inaccurate optical center of gravity of the beam image formed on a semiconductor position sensitive device (PSD), and thus correct object distance information is not obtained.
The configuration of a conventional distance measuring device is shown in FIG. 4. A beam of light having a substantially circular cross section is emitted from an IRED 212 and condensed and projected toward an object 100 to be photographed. The light reflected by the object 100 is condensed by a light receiving lens 221 and is imaged as a light spot on a PSD 222. If the object 100 to be photographed is in a position slightly shifted to the right from the center of a photographic field as shown in FIG. 5(a), the light spot on the PSD 222 is not substantially circular and only a part of the circle is imaged on the PSD 222. At this time, the optical center of gravity of the light spot should be in a position displaced from the center of the PSD 222 by a value L, but it is actually displaced by an additional length of .DELTA.L due to a beam loss. As a result, the position is determined to be farther than the actual position of the object. If the object 100 to be photographed is in a position slightly shifted to the left from the center of the photographic field, the position is determined to be closer than the actual position of the object in the same way. In either case, the photographic lens is driven so that it focuses at a position different from the actual distance to the object to be photographed. This results in a photograph that is out of focus and blurry.
Various conventional methods attempt to overcome the distance error resulting from this partial loss of the reflected light beam, referred to as "beam loss". For example, Japanese Patent Publication No. H3-2611 discloses a distance measuring device comprising a combination of a light projecting element and a light receiving element. However, when assembled on a camera, this device fails to provide a large enough distance between the light projecting and light receiving elements, due to spatial limitations, especially camera width, to be effective in measuring long distances. This reduces the accuracy of the distance measurement obtained when the distance to the object to be photographed is long. Japanese Patent Publication No. H1-288714 discloses a distance measuring device comprising a plurality of light emitting elements and a plurality of light receiving elements. This configuration results in a complicated and expensive device due to the required number of costly light emitting elements and light receiving elements used.